A phase change material is a material that changes into a crystalline state or into an amorphous state in response to changes in temperature. The specific resistance of the phase change material in the crystalline state may be lower than that in the amorphous state. This feature of the phase change material may allow the phase change material to be used in a memory device, for example, a phase-change random access memory (PRAM).
A unit cell of a PRAM includes an access device and a phase change resistor. The phase change resistor generally includes a phase change material layer disposed between a bottom electrode and a top electrode, and the access device is connected to the bottom electrode to transmit a writing current applied to the top electrode.
FIG. 1 presents a graph that demonstrates a method of performing a set or reset programming operation in a phase change resistor. Specifically referring to FIG. 1, a phase change material layer in an amorphous state is heated to a temperature between a crystallization temperature (Tx) and a melting point (Tm) and then cooled. Thus, the phase change material layer is changed from the amorphous state to a crystalline state (set programming). In contrast, when the phase change material layer is heated to a temperature higher than the melting point (Tm) and the abruptly cooled, the phase change material layer is changed from a crystalline state to an amorphous state (reset programming).
In this instance, the heating temperature applied to the phase change material is adjusted by the amount of a writing current that flows through the bottom electrode and the access device. In other words, when a writing current flows through the bottom electrode and the access device, joule heat is generated at an interface between the bottom electrode and the phase change material layer, and the temperature based upon the joule heat can be determined by the amount of the writing current.
During reset programming, in order to apply a relatively high current, the size of the access device may be enlarged; however, this action can present an obstacle for increasing the device integration. In order to address such a problem, ways of reducing the contact surface area between the bottom electrode and the phase change material layer have been researched in order to increase the effective current density of the writing current. For example, the contact surface area between the bottom electrode and the phase change material layer can be reduced by forming fine via holes exposing fine regions of the bottom electrode and then filling the via holes with a phase change material. However, the phase change material layer is typically formed using a sputtering method, and thus, it may be problematic to fill the via holes without voiding at least due to step coverage of the phase change material layer formed using the sputtering method.
Korean Patent Laid-Open Gazette No. 2006-0008027 discusses a technique of forming a phase change layer in a contact hole using a chemical vapor deposition (CVD) method. In detail, at 700° C., GeH4 is used as the Ge source, one of Sb(C2H5)3, Sb(C3H7)3, Sb(CH3)3, Sb(C2H3)3, and Sb4 is used as the Sb source, and one of Te(C2H5)3, Te(C2H3)3, Te(CH3)3, Te(C4H9)2, Te(CH3)(C3H5), and Te(C3H5)2 is used as the Te source to form a Ge2Sb2Te5 phase change material layer. However, it is generally problematic to fill fine contact holes with the phase change material layer at least because of the size of grains of the phase change material layer formed at a relatively high temperature of 700° C.